Polyfunctional amines are used as coreactants with polyfunctional isocyanates in many applications. These reactions yield polyureas which are a highly regarded class of polymers, known for their toughness, high strength and dynamic mechanical and high temperature performance. These reactions are generally carried out in specialized equipment, due to the high reactivity of amines and isocyanates. This has limited their use in traditional adhesives, coatings and sealer type product applications.
Typical primary and secondary amines are extremely reactive with a variety of electrophiles, which results in very short gel times with little or no potlife. The reaction of primary amines with isocyanates is extremely exothermic and produces strongly hydrogen bonded dihydro-urea linkages. Strongly hydrogen bonded groups may disadvantageously increase product viscosities and hinder subsequent mobility and reactivity of attached functional groups. Additionally, uncontrollable reaction rates are undesirable in many applications.
The utility of amines of the present invention is based on the addition reaction between a polyisocyanate component and an isocyanate-reactive component, in particular polyamines containing secondary amino groups. This reaction is known in principle from DE-OS 2,158,945 (Fed. Rep. Of German, 1973) but according to the teachings of this publication the reaction is not used for crosslinking two-component coating compositions at relatively low temperatures but rather for the preparation of intermediate products which are converted at elevated temperatures into heterocyclic end products.
Bayer Corp. has recently introduced polyaspartic acid diesters under the name DESMOPHEN.TM.. These secondary amines react more controllably with electrophiles than do the corresponding primary amines. However, the adducts of these amines and isocyanates are capable of a further transformation to form a hydantoin ring structure, giving rise to shrinking of the coating and generating undesired alcohol byproducts.
U.S. Pat. No. 5,126,170 describes secondary amines, referred to as "polyaspartic acid derivatives", which are formed by the Michael-type reaction of primary polyamines with maleic or fumaric acid ester Michael receptors. This reaction is illustrated below as formula (I), wherein R.sub.1, R.sub.2 and R.sub.3 are as defined in the cited reference. ##STR1## These aspartic ester diesters react with isocyanates to form urea-diester linkages. This reaction is illustrated below as formula (II), wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are as defined in the cited reference. ##STR2##
Urea-diester linkages are reportedly unstable however, cyclizing to the hydantoin with the concomitant expulsion of alcohol. The expelled alcohol is problematic in some systems such as isocyanate-terminated prepolymers, since it may undesirably react with residual isocyanate groups. Dimensional changes of the polymer upon hydantoin formation is a potential problem as well. This reaction is illustrated below as formula (III), wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are as defined in the cited reference. ##STR3##
In addition to U.S. Pat. No. 5,126,170, the above aspartic ester diesters are described to be suitable for manufacture of high performance polyurethane or polyurea coatings for specific end-use applications in U.S. Pat. Nos. 5,236,741, 5,243,012, 5,412,056, 5,516,873 and 5,580,945.
There is a need for improved polyurea and polyurethane coatings having amine components that resist hydantoin formation.
There is a significant need for a liquid pavement marking composition that will provide increased durability and retained reflectivity once applied to a surface and dried or cured. Furthermore, it is advantageous to apply markings in a wider range of weather conditions than is possible with existing compositions. There is also a need for marking compositions with improved cure profiles to ensure both substrate wet out and rapid track free time. Improvements are needed to obtain compositions that are substantially free of volatile organic components. Compositions of this type are typically used on roads, highways, parking lots and recreational trails to form stripes, bars and markings for the delineation of lanes, crosswalks, parking spaces, symbols and legends and the like. They are typically applied by spray coating (i.e., painting) the pavement surface. Preformed pavement marking sheets or tapes have also been used to mark pavement or traffic bearing surfaces.
Pavement marking stripes, or pavement markings of other shapes, may include reflective optical elements adhered to the pavement surface by the use of a binder. Current traffic paint systems typically use conventional 1.5 n.sub.D glass microspheres for increased retroreflection. The microspheres are typically flood coated onto the wet marking immediately after coating. This provides the paint with improved retroreflectivity and also covers the top surface of the uncured or undried coating with a protective layer of microspheres. This protective layer allows the markings to be exposed to traffic sooner because of the layer of microspheres over the surface, which prevents transfer of the coating to the surface of vehicle tires. This is important for rates of marking application. The time between application and the point where material will no longer transfer to vehicle tires is defined as the "track free" time. Shorter track free times increase marking efficiency by reducing or eliminating the need for traffic disruption through such measures as closing lanes or placing traffic control devices to protect such markings.
U.S. Pat. No. 5,478,596 discloses liquid pavement marking compositions that solve many of the problems of alkyd-based and epoxy-based pavement marking compositions. Such pavement marking compositions are prepared from a two part polyurethane-forming system of a first component having isocyanate-reactive groups (a polyol) and a second component having isocyanate groups. It is disclosed that such compositions dry faster, withstand weathering better, and do not discolor as readily as alkyd-based and epoxy-based compositions. However, the exemplified compositions required aromatic isocyanates in combination with the polyols and a catalyst. Any colorless or lightly colored aromatic polyisocyanate is suggested to increase the reactivity and decrease the viscosity of the isocyanate component and provide a harder polyurethane. Aromatic isocyanates are not particularly desirable because the resultant polyurethanes are subject to environmental degradation and discoloration. Also, many lower molecular weight aromatic isocyanates that would decrease viscosity and modify reactivity or film properties can potentially pose significant inhalation risk or toxic hazard related to their vapor pressure. Furthermore, the use of catalysts is not desirable because they can also catalyze degradation of the polyurethanes. To overcome the deficiencies of aromatic isocyanates, aliphatic isocyanates could be used with polyols; however, this would require the use of a catalyst or an aromatic isocyanate together with an aliphatic isocyanate to obtain sufficient rates of cure.
Thus, the need still exists for liquid pavement marking compositions that can simultaneously provide all of these features in a single material: reduced environmental impact through formulations having low volatile organic content or that are substantially solvent-free; improved balance of coating rheology during application and film formation to promote substrate wet out and fast cure to track-free films; broadened range of weather conditions for coating application; and improved marking performance through increased durability and retained reflectively. Especially needed are liquid pavement marking compositions that resist shrinkage, thus avoiding cracking, coating imperfections and failures.